1. Field of the Invention
The present invention relates to a non-contact type IC card which contains a power source for data communication with an external device such as a terminal device or the like using electromagnetic waves.
2. Description of the Related Art
FIG. 12 is a block diagram showing a conventional non-contact type IC card. In the drawing, reference numeral 1 denotes the body of an IC card. The IC card body 1 comprises a CPU 2 for controlling signals and processing data in the card, a ROM 3 for storing a program therein, a RAM 4 serving as a data memory, a battery 5, an input/output circuit 6 for parallel-serial conversion/serial-parallel conversion, and an antenna 10.
The CPU 2, the ROM 3, the RAM 4 and the input/output circuit 6 are connected to each other through a bus 7 so that data can be transmitted to and received from each other. A modulator/demodulator 8 is interposed between the transmitting circuit 9 and a receiving circuit 11 are interposed between the modulator/demodulator 8 and the antenna 10. The data output from the input/output circuit 6 is modulated by the modulator/demodulator 8, passed through the transmitting circuit 9 and then output as an electromagnetic wave from the antenna 10. The data received by the antenna 10 is amplified by the receiving circuit 11. A trigger signal is input directly to the CPU 2, and the data signal is modulated and converted into a logic level by the modulator/demodulator 8, passed through the input/output circuit 6 and the bus 7 and input to the CPU 2 in which the signal is processed. The battery 5 serves as a power source for each of the circuits. The connection of the battery 5 is not shown in the drawing.
The operation of the conventional IC card is described below. Although electricity is usually constantly supplied to the receiving circuit 11 and so on from the battery 5, when the IC card does not communicate with an external device such as a card reader or the like, an oscillation circuit in the CPU 2 is stopped so that the IC card assumes a stand-by state. When a trigger electromagnetic wave for activating the CPU 2 is output from the external device and is received by the antenna 10, an analog signal corresponding to the electromagnetic wave received is output to the receiving circuit 11 from the antenna 10. If the voltage of the signal exceeds the threshold voltage of the receiving circuit 11, the receiving circuit 11 generates a trigger signal at the logic level and inputs it to the CPU 2. When the trigger signal is input to the CPU 2, the oscillation circuit in the CPU 2 is operated, and the data signal is converted into a digital signal by the receiving circuit 11, demodulated by the modulator/demodulator 8 and then input to the input/output circuit 6. The data input to the input/output circuit 6 is read by the CPU through the bus 7. An operation clock signal supplied to each of the circuits, and the data signal is processed according to the program stored in the ROM 3. During data processing, data is temporarily stored in the RAM 4. The receiving circuit 11 comprises a differential amplifier circuit or the like and has a threshold voltage of several hundreds of mV.
On the other hand, during or after data processing, transmit data to be transmitted to the external device is input to the input/output circuit 6 through the bus 7. The input/output circuit 6 inputs the data as a digital signal to the modulator/demodulator 8 which modulates the digital signal and inputs it to the transmitting circuit 9. The transmitting circuit 9 converts the digital signal to an analog signal and outputs the analog signal to the antenna 10 which outputs as an electromagnetic wave the signal to the external device.
FIG. 13 shows the output signal of each of the antenna 10, the receiving circuit 11 and the modulator/demodulator 8. The antenna 10 inputs the analog signal shown in FIG. 13A to the receiving circuit 11. In the receiving circuit 11, for example, a threshold value V.sub.TH is set so that the analog signal is converted into the digital signal shown in FIG. 13B on the basis of the threshold value. The digital signal is sent from the receiving circuit 11 to the modulator/demodulator 8, demodulated to the digital signal shown in FIG. 13C by the modulator/demodulator 8 and then sent to the input/output circuit 6. When a single pulse is missing in a pulse group, as shown on the right side of the drawing, pulses are generated so as to make up the pulse missing. Conversely, when the IC card sends a signal from the antenna 10, the digital signal shown in FIG. 13C which is sent from the input/output circuit 6 is modulated to become the signal shown in FIG. 13B by the modulator/demodulator 8 and converted into the analog signal shown in FIG. 13A by the transmitting circuit 9. The analog signal is supplied to the antenna 10 and then sent to the external device.
When communication with the external device is completed, the oscillation circuit of the CPU 2 is again stopped so as to bring the IC card into a stand-by state, thereby suppressing power consumption. However, in the stand-by state, a bias current is constantly passed through the receiving circuit 11 for receiving the trigger electromagnetic wave.
The power consumption of such a type of IC card includes a constantly flowing bias current and a large current of several mA which is consumed for starting the operation of the CPU 2 then the intensity of the electromagnetic wave received by the antenna 10 exceeds the threshold voltage of the receiving circuit 11.
However, such a voltage above the threshold voltage is generated not only by the trigger electromagnetic wave output from a predetermined external device but by the noise generated by other electronic devices which sometimes activates the IC card in the same way as that described above. In an environment generating much noise, the battery is therefore consumed before the end of the guarantee period due to the frequent activation of the CPU 2. This battery failure makes data loss or the like.
In addition, the battery 5 has a small capacity because the IC card body 1 has a thin plate-like form, and the battery 5 cannot be replaced by a new battery because the battery 5 is sealed by the outer package provided on the IC card body 1. The conventional IC card has the above-described problems.